I'm learning that if you want to truly be happy you have to be with people who will accept you for who you are. Now that I know I am officially poly, any guy I date has to know and accept that this is what I am, and that I will not change in a relationship.
You seem to be dating people who won't accept that aspect of you? I'm experiencing firsthand how much hurt can come from that.
When I realised I am poly my ex wouldn't accept it, and while we still both care deeply about each other and have admitted as much, and are there for each other as friends, we are at a complete stalemate and know that we can never be together. It hurts a lot.
Please do yourself a favour and tell people you are interested in upfront that you are poly. A lot of hurt comes from not being open about that, especially once deep feelings are admitted